The present invention relates to an oilfield hanger and wellhead system and more particularly to a hanger assembly for suspending a tubular string in tension from a surface wellhead on an offshore platform.
Offshore drilling and production systems include a subsea wellhead system or mud line suspension system for supporting concentric tubular pipe strings such as casing and tubing strings into the borehole of an offshore well. A drilling platform is located at the water's surface such as on a jack up rig, a floating rig, or a tension leg platform, as for example. Risers or tie-back casing strings extend from the mud line suspension system to a surface wellhead system located at the drilling platform to tie the sub sea wellhead with the surface wellhead. The riser or tie-back casing string engages a hanger at the mud line at its lower end and is suspended by another casing hanger at its upper end at the surface wellhead. Typically the outer casing string is a drilling riser which includes one or more concentric tie-back casing strings suspended therewithin. One or more production tubing strings are ultimately suspended within the concentric casing strings.
The surface wellhead includes a high pressure housing for supporting casing and tubing strings and controlling downhole pressure. An annulus is formed between the outer drilling riser and the inner tie-back casing string and attachments may be provided on the housing to control the annulus pressure if required. The surface wellhead includes metal-to-metal seals to prevent leakage and provides backup seals spaced from the metal-to-metal seals to monitor leakage through monitoring ports.
It is desirable that the casing and tubing strings are suspended in tension. A floating platform will heave due to the swells and waves of the water, thus raising and lowering the elevation of the platform. Further, the heat generated by the flow of hydrocarbons through the inner production tubing string will cause linear expansion of the outer casing strings which might otherwise make them buckle due to the induced expansion and contraction.
The outer drilling riser may include a pipe section adjacent to its upper end with outer grooves for engagement with and connection to a hydraulic lift mounted on the drilling platform. One type of hydraulic lift includes a rocker arm which has a connector at its terminal end with a plurality of teeth for mating engagement with the grooves on the upper pipe section of the drilling riser. The grooved pipe section is several feet long to avoid a precise alignment of certain grooves with the teeth of the hydraulic lift. Once the rocker arm is attached to the grooved pipe section, the drilling riser is placed in tension by hydraulically actuating the rocker arm to elevate the drilling riser.
Tie-back casing strings and production tubing strings are suspended within the outer drilling riser. These strings are suspended in tension by a surface hanger. For example, tie-back casing strings are connected at their lower ends to other casing strings at the mud line which are suspended within the borehole by the mud line suspension system. The majority of the load of the casing string suspended into the borehole is supported at the ocean floor by a conventional subsea wellhead. The tie-back casing string at the surface wellhead may support a portion of the casing load at the sub sea wellhead as well as the weight of the tie-back casing string. Current applications in 3,000 feet of water using seven inch, 32 pounds per foot casing, will generate approximately 7,500 per inch of linear stretch measured at the surface. This assumes that the total landed surface load exceeds the cumulative weight of the tie-back casing string extending between the surface wellhead and the subsea wellhead. These factors combined with installation limits in controlling the measured space-out, dictate that the surface hanger for the tie-back casing string have the ability to accommodate some variation in the final landed elevation of the surface hanger. Thus, a variable position style surface hanger is required due to the inability to accurately measure and space-out the tie-back casing string between the subsea wellhead and the surface wellhead. This difficulty in spacing out long tie-back casing strings requires that the surface hanger be able to adjust up or down a substantial distance, such as, for example, up to four feet.
Different methods have been employed to suspend a tubular string in tension from the surface wellhead. One method is to adjust the surface hanger up or down to achieve proper tensioning and then cut the hanger or tubular pipe to the proper length. Another method is to make the surface wellhead large enough to receive the entire surface hanger and pack off. This works well for small adjustments, such as 2 to 4 inches, but not when an adjustment of a matter of feet is required. Still another method is to move the entire wellhead up or down as necessary, but this is very expensive and requires the drilling rig to have unlimited height capability causing this method to be ineffective.
A slip hanger may be used to support the string in tension at the surface wellhead. Upon the string being held in tension, slips, in the form of arcuate wedges, are disposed between the pipe string and the surface wellhead. The slips include threaded surfaces which bite into the outer cylindrical surface of the tubular pipe. The slip hangers allow the use of a predetermined tension on the string since they bite into the pipe at any elevation to achieve the desired tension. However, certain problems may arise in the use of a slip hanger. The use of a slip hanger is a time consuming operation. Further, slip hangers are imprecise in the amount of tension maintained since slippage can occur as the slips are installed. Further, in tension leg platforms, there is greater heave of the platform caused by the water and also there is more stretch in the pipe string particularly at greater water depths. In such situations, the biting and indenting of the slips into the outer surface of the pipe enhances the fatigue factor of the pipe due to the substantial dynamic load caused by the constant wave action and heave of the platform.
U.S. Pat. No. 4,938,289 discloses a surface wellhead which includes a hanger assembly having a hanger threaded to an annular support sleeve for suspending a casing string in tension. An upward force is applied to the casing string to tension and stretch the casing, thereby raising the upper end of the casing above a landing shoulder. When the desired tension has been applied, an actuator sleeve, engaging the annular support sleeve, is rotated causing the annular support sleeve on the hanger to rotate and move downwardly on its threaded connection with the hanger until the annular support sleeve lands on the landing shoulder of the wellhead. The applied tension from the casing string is then released with the tension being maintained by the engagement of the annular support sleeve on the landing shoulder.
Another type of conventional tie-back apparatus is the tension integral tie-back system of Cooper Industries, Inc. which includes a two-piece tie-back sub that is installed in the casing string just below the surface hanger. When the surface hanger is landed, a forging tool is run on drill pipe and positioned into the tie-back sub. Once the forging tool is installed, a set of lifting segments is mechanically engaged into prepared slots and tension is applied. While this tension is being maintained on the tie-back string, hydraulic pressure is applied to the forging tool, producing a downward thrust on a tapered plug within the forging tool. This plug engages forging dies forcing them out to deform an inner sleeve into a pre-machined profile in the outer sleeve of the sub.
The hanger assembly of the present invention overcomes the deficiencies of the prior art and provides for varying the landing position of the hanger and installing a pack off to isolate the pressures above and below the connection.